The present invention relates to a portable visual display apparatus and, more particularly, to a head- or face-mounted visual display apparatus that can be retained on the observer's head or face.
A head-mounted visual display apparatus such as that shown in the plan view of FIG. 22 has heretofore been known (see U.S. Pat. No. 4,026,641). In the conventional head-mounted visual display apparatus, an image of an image display device 46, e.g., a CRT, is transferred to an object surface 12 by an image transfer device 25, and the image transferred to the object surface 12 is projected in the air by a toric reflector 10.
As another related art, a head-mounted visual display apparatus that uses a decentered concave ocular optical system and a decentered relay optical system is disclosed in Japanese Patent Application No. 03-295874 (1991) by the present applicant. FIG. 23 is a sectional view of one embodiment of the prior head-mounted visual display apparatus. In the figure, reference symbols denote elements or portions as follows: P is the axis of rolling of an observer's eyeball 13; C is the observer's visual axis lying when he or she sees forward; Q.sub.1 is the position of the observer's pupil; S.sub.8 is a spheroid having T as an axis of revolution; 16 is a reflecting surface of the spheroid; 17 is an optical axis of a relay optical system; Q.sub.2 is the focal point of the spheroid; 15 is the relay optical system; and 14 is a two-dimensional image display device.
For a head-mounted visual display apparatus, it is important to minimize the overall size and weight thereof in order to make the observer feel comfortable when wearing it. An essential factor in determining the overall size of the apparatus is the layout of the optical system.
In the case of a direct-vision layout in which an enlarged image of a two-dimensional image display device is observed directly through a convex lens, the amount to which the apparatus projects from the observer's face is unfavorably large. Further, it is necessary in order to provide a wide angle of view to use a large positive lens system and a large two-dimensional image display device. Accordingly, the apparatus inevitably becomes large in size and heavy in weight.
To enable the observer to use the apparatus for a long time without fatigue and to attach and detach the apparatus with ease, it is preferable to adopt an arrangement in which an ocular optical system including a reflecting surface is disposed immediately in front of the observer's eyeball. With this arrangement, a two-dimensional image display device, an illuminating optical system, etc. can be disposed in a compact form around the observer's head. Thus, it is possible to reduce the amount of projection of the apparatus and also the weight thereof.
Next, it is necessary to ensure a wide angle of view in order to enhance the feeling of being at the actual spot which is given to the observer when viewing the displayed image. In particular, the stereoscopic effect of the image presented is determined by the angle at which the image is presented (see The Journal of the Institute of Television Engineers of Japan Vol. 45, No. 12, pp. 1589-1596 (1991)).
The next matter of great concern is how to realize an optical system which provides a wide angle of view and high resolution.
It is known that it is necessary in order to present a stereoscopic and powerful image to the observer to ensure a view angle of 40.degree. (.+-.20.degree.) or more in the horizontal direction, and that the stereoscopic and other effects are saturated in the vicinity of 120.degree. (.+-.60.degree.). In other words, it is preferable to select an angle of view which is not smaller than 40.degree. and which is as close to 120.degree. as possible. However, in a case where the above-described ocular optical system is a plane reflecting mirror, it is necessary to use an extraordinarily large two-dimensional image display device in order to make light rays incident on the observer's eyeball at a view angle of 40.degree. or more. After all, the apparatus increases in both the overall size and weight.
Further, since a concave mirror produces strong curvature of field along the surface of the concave mirror because of its nature, if a planar two-dimensional image display device is disposed at the focal point of a concave mirror, the resulting observation image surface is curved, so that it is impossible to obtain an image for observation which is clear as far as the edges of visual field. There is a method wherein the display surface of a two-dimensional image display device is curvedly disposed, as in the case of the prior art shown in FIG. 22. However, even when a two-dimensional image display device is disposed at the front focal point of a concave mirror so that an image of the two-dimensional image display device is projected in the air as an enlarged image by only a concave mirror, as in the arrangement shown in FIG. 22, it is difficult to obtain high resolution because of the aberration of the concave mirror when a view angle of 40.degree. or more is provided.
When a decentered correcting optical system is used as in the arrangement shown in FIG. 23, since the decentered correcting optical system lies in the vicinity of the observer's face, the user cannot observe the displayed image with his/her spectacles or the like on. The reason for this will be clear from FIG. 23. That is, a frame portion of the spectacles interferes with the decentered correcting optical system, and light rays from the relay optical system, which forms an image for observation, strike a spectacle lens from the rear side thereof. Therefore, it is impossible to observe a normal observation image.
In view of the above-described problems of the background art, it is an object of the present invention to provide a visual display apparatus which provides a wide angle of view, and yet which is small in size and light in weight and has high resolution and a large exit pupil diameter, and also provide a visual display apparatus which has the above-described advantageous features, and yet which enables the user to observe a displayed image with his/her spectacles on.
The purpose of providing a large exit pupil diameter in the present invention will be explained below. If the exit pupil diameter of the optical system is not sufficiently large, the visual field is eclipsed by the rolling movement of the eye when the user tries to observe a peripheral region of the visual field. The way in which eclipse occurs is shown in FIGS. 4(a) and 4(b). In FIG. 4(a) a state wherein the position of the pupil 2 of the observer's eye 1 when observing the center of the visual field is coincident with the position of the exit pupil of the optical system is depicted. On the other hand, FIG. 4(b) shows a state wherein, to observe a peripheral region of the visual field, the observer rolls his/her eye 1 toward it. In this case, since the observer's pupil 2 and the rolling axis of the eye 1 are displaced from each other, it seems as if the pupil 2 were transversely offset. For this reason, when the observer turns his/her eye 1 leftward to observe the left, for example, the right-hand side visual field is eclipsed and becomes invisible.
Further, the positional relationship between the observer's pupil and the exit pupil of the apparatus may change according to the condition in which the apparatus is attached to the observer's head or face. Unless the apparatus has an exit pupil diameter which is larger than the observer's pupil diameter to a certain extent, it is impossible to absorb a displacement between the observer's pupil position and the exit pupil position of the apparatus, which occurs according to the condition in which the apparatus is mounted or the difference in physical size and shape among individuals. Consequently, the image for observation is partly cut off by the observer's pupil, and a wide angle of view cannot be ensured.
To solve the above-described problem, it is essential to design the observation optical system so that it has a sufficiently large exit pupil diameter. It is difficult even in ordinary camera lenses to increase the pupil diameter, that is, to reduce the F-number, from the viewpoint of correcting aberrations of the lens. Therefore, it is extremely difficult to double the pupil diameter. For example, although a standard lens for camera that has an F-number of 2.8 and a focal length of 50 mm is of the triplet type, which is made of three lens elements, a standard lens for camera having an F-number of 1.4 needs a Gauss type lens system, which includes six lens elements. Thus, to double the pupil diameter (that is, to halve the F-number), the arrangement of the optical system must be changed to a considerable extent, which invites an increase in size of the lens arrangement.
Incidentally, about half of the general people suffer from visual disorders such as myopia, astigmatism, etc. The proportion of contact lens users to the total number of people using glasses has been increasing in recent years. However, the use of contact lenses is still limited to some people because of the troublesomeness in handling the lenses or conservatism, and most people suffering from visual disorders still use spectacles in view of price, handling and so forth.
To enable a person who uses glasses to view a clear image for observation without glasses by using a visual display apparatus such as that shown in FIG. 22 or 23, for example, some diopter correcting device must be provided on the visual display apparatus side.
However, when a device for correcting diopter, including astigmatism, is provided on a visual display apparatus such as that of the present invention, which aims at reducing the overall size and weight of the apparatus, various problems arise. That is, the size and weight of the apparatus are increased, and it is extremely difficult to enable the amount of correction of diopter, which is made on the apparatus side, to be appropriately adjusted so that the corrected diopter matches the observer's eyes. When a user who has normal eyesight observes for a long time with a wrong diopter, the observer's diopter adjusts to the wrong diopter on the apparatus side, which involves the danger of the observer's eyesight becoming poor.
Further, to perform "superimpose" observation in which an aerial image of a two-dimensional image display device and an observation image in the outside, real world are viewed superimposed on one another, a diopter correcting mechanisms must be added for both the outside world image and the aerial image, which is projected in the air by the visual display apparatus, resulting in a further increase in size of the apparatus.